Leishmaniasis is a cosmopolitan disease causing substantial morbidity and mortality in much of the developing world. The clinical manifestations of Leishmania infection vary substantially ranging from self-healing lesions to fatal visceral disease and are attributed to differences in the Leishmania species initiating the infections. The fact that these parasites exhibit such different virulence characteristics makes this group an excellent model to evaluate the precise contributions that host and parasite related properties play in generating different pathologies. Animal models studying resistance to Leishmania indicate protective immunity requires the generation of CD40/CD40L-dependent IL-12 generated Th 1 responses. The initIal events that occur during an infection set the tone for a subsequent immune response. To model the initial events of Leishmania infection, we employ an in vitro system of human dendritic cells (DC) and infectious stage parasites. We previously demonstrated that the induction of IL-12 by these cells is parasite species dependent and requires CD40L stimulation. Whereas infections with species responsible for visceral disease (L. donovani) cause no upregulation of CD40L-dependent IL-12 production, infection with L. major, a species associated with self-limiting cutaneous disease, primes dendritic cells for augmented IL-23 secretion. The overall goal of this proposal is to identify the molecular determinants responsible for these disparate IL-12 responses. Two possibilities exist to explain these differences: 1) L. major parasites have some intrinsic capacity to prime human DC for IL-12 production, or 2) microbial stimulus in general primes for IL-23 and L. donovani parasites induce some down-regulatory cascade to inhibit CD40L stimulation of IL-12. To test these hypotheses, the following aims are proposed: Specific Aim 1. To identify DC secreted products induced by Leishmania that play a role in IL-12 regulation. The induction of factors induced by Leishmania parasites and their inhibitory or enhancing capacities on IL-12 production will be assessed. Specific Aim 2. To determine the point of regulation that Leishmania spp. modulate IL-12 production in human DC. Studies will be performed to determine if the block on IL-12 secretIon in L. donovani- infected DC involves transcriptional or translational regulation. Specific Aim 3. Identification of the factor(s) that regulate IL-12 induction by Leishmania. Leishmania binding to and invasion of host cells is mediated via specific host cell surface receptors. The role of these specific receptors in Leishmania priming of human DC for IL-12 production will be assessed. Studies also will be initiated to identify the parasite factors involved in this process. The intrinsic differences in the ability of Leishmania to prime dendritic cells for IL-12 production partially may account for the evolution of healing and non-healing forms of leishmanial disease and may serve as novel antigens or chemotherapeutic targets to combat this devastating disease.